Available data suggest that if energy expenditure (EE) is involved in the etiology of obesity, the component of interest is activity-related EE (AEE). Building on our previous studies, we propose to use a carefully controlled model of the post-obese state by reducing obese black and white women to a normal-weight post-obese state, and by comparing them with never-obese controls. In these groups we will test the hypotheses that 1) reduced spontaneous AEE is a causative factor, rather than an effect, of the obese state, and that 2) the cause of reduced AEE is an underlying disorder of muscle substrate oxidation which, in turn, causes increased physiologic and perceived exercise difficulty in obese and post-obese persons, compared to never-obese controls.